The present invention relates to a semiconductor laser device and to a method for fabricating the same. More particularly, it relates to an increase in the output of the semiconductor laser device.
Recent years have seen rapid widespread use of DVD (Digital Versatile Disk) devices in the fields of AV (Audio-Video) equipment, PCs (Personal Computers), and the like. In particular, great expectations have been placed on the use of recordable DVD devices (such as DVD-RAM and DVD-R) as large-capacity memory devices embedded in PCs and the like and as post-VTR (Video Tape Recorder) devices.
As pickup light sources for the foregoing DVD devices, red semiconductor lasers at wavelengths in the 650 nm band have been used. With the recent increases in the density and capacity of an optical disk, a pick-up light source capable of performing a particularly high output operation over 80 mW has been in growing demand to allow a higher-speed write operation with respect to the optical disk.
If a semiconductor laser device is increased in output, however, the problem is encountered that the output thereof is saturated. The problem is conspicuous when the semiconductor laser device is operating at a high temperature. This is because an AlGaInP-based material composing the p-type clad layer of the red laser has a limitation in that it cannot provide a sufficiently large band barrier against electrons in the conduction band. Under high-temperature and high-output operating conditions, therefore, overflowing electrons are increased and a component of an injected current which does not contribute to light emission is increased.
In suppressing the overflow of electrons, it is effective to increase the concentration of a p-type impurity in the p-type clad layer and enlarge the band barrier against the electrons. i.e. accomplish high-concentration doping of the p-type clad layer with a p-type impurity.
However, the influence of solid-phase diffusion cannot be ignored in the AlGaInP-based material because of the diffusion coefficient of the p-type impurity which is generally high therein. It has been reported that the diffusion coefficient of Zn, e.g., in an AlGaInP-based material is 1×10−13 to 6×10−13 cm2s−1, which is about two orders of magnitude higher than that of Zn in an AlGaAs material used for the infrared laser. The influence of solid-phase diffusion becomes more conspicuous in the case of high-concentration doping since the diffusion coefficient of the impurity is directly proportional to the square of a doping concentration.
FIG. 3 shows a SIMS profile of an active layer and a clad layer in a conventional semiconductor laser device. As shown in FIG. 3, Zn has been mixed in the active layer though it has not been doped with an impurity intentionally. This results from the diffusion of an impurity used to dope the p-type clad layer in the active layer during the processes of crystal growth, thermal treatment, and the like for fabricating the semiconductor laser device. Similar impurity diffusion also occurs during the operation of the semiconductor laser device. If the impurity is diffused in the active layer, a nonradiative recombination center may be formed to degrade the characteristics of the semiconductor laser device. The production of a crystal defect reduces the lifespan of the semiconductor laser.
Due to the impurity diffusion in the active layer described above, the doping of the p-type clad layer with the p-type impurity at a high concentration has been difficult.
As a method for suppressing impurity diffusion in the active layer, therefore, Japanese Unexamined Patent Publication No. 2000-286507 discloses one which provides an undoped spacer layer between the p-type clad layer and an optical guide layer such that an impurity is absorbed in the spacer layer. If the doping concentration of the p-type clad layer of an AlGaInP-based semiconductor laser device is to be increased by using the method, the spacer layer for suppressing impurity diffusion should have a sufficiently large thickness. As the thickness of the spacer layer is increased, however, a series resistance is increased so that an operating current is increased disadvantageously or it becomes difficult to obtain a desired beam configuration under the influence of the spacer layer.